JEE Main & Advanced Mathematics Applications of Derivatives Question Bank Self Evaluation Test - Application of Derivatives

  • question_answer
    If the sub-normal at any point on \[y={{a}^{1-n}}{{x}^{n}}\] is of constant length, then the value of n is

    A) \[\frac{1}{4}\]

    B) 1    

    C) \[\frac{1}{3}\]

    D) \[\frac{1}{2}\]

    Correct Answer: D

    Solution :

    [d] \[y={{a}^{1-n}}{{x}^{n}}\] or \[\frac{dy}{dx}={{a}^{1-n}}n{{x}^{n-1}}\] \[sub-normal=\left| y\frac{dy}{dx} \right|=\left| y{{a}^{1-n}}n{{x}^{n-1}} \right|\] \[=\left| {{a}^{1-n}}{{x}^{n}}{{a}^{1-n}}n{{x}^{n-1}} \right|=\left| {{a}^{2-2n}}{{x}^{2n-1}} \right|\] which is constant if \[2n-1=0\] or \[n=1/2\].


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